This invention relates to an open architecture superconducting magnet cryogen vessel structure suitable for magnetic resonance imaging, and in particular to the magnetic components assembly within the cryogen vessel.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils can be made superconducting, such that when a power source is initially connected to the magnet coils (for a relatively short period) current continues to flow through the coils even after power is removed due to the absence of resistance, thereby maintaining a strong magnetic field. Superconducting magnets find wide application in the field of Magnetic Resonance Imaging (hereinafter MRI).
Another problem encountered by conventional and early MRI equipments is that they utilize solenoidal magnets enclosed in cylindrical structures with a central bore opening for patient access. However, in such an arrangement, the patient is practically enclosed in the warm bore, which can induce claustrophobia in some patients. The desirability of an open architecture magnet in which the patient is not essentially totally enclosed has long been recognized. Unfortunately, an open architecture poses a number of technical problems and challenges.
One type of open architecture superconducting magnet utilizes a split dewar or split liquid helium vessels with the lower helium vessel and the upper helium vessel connected by one or more supports on one end of the vessels.
Such open architecture superconducting magnets may utilize or require iron rings to shape and confine the magnetic field. The field strength of such magnets affects the signal-to-noise ratio and imaging quality. However, open architecture superconducting magnets provide a generally low field strength as compared with more conventional cylindrical magnets.
Problems encountered as a result of the incorporation of iron rings include the interaction of the strong magnetic field of the superconducting magnet coil with the iron rings. In addition, the thermal differential expansion and contraction of the iron ring and the magnetic components during the temperature cycles of over 400.degree. F. encountered in ramping up the magnet to superconducting operation, or the warm up after discontinuance of superconducting operation, present difficulties in the design of a suitable magnet structure. Still further, the magnet assembly must support and maintain accurate spacing between the iron ring and the magnet coils.
It is also highly desirable to maximize the patient space between the superconducting magnet coils closest to the imaging region while at the same time providing adequate support for those magnetic coils.
Thus, difficulties are presented in maintaining imaging quality, and in maintaining precise magnetic components support and fixed relative positioning in the presence of such severe and conflicting environmental and operational considerations.